More about java strings - Immutability and String Pool

More about java strings - Immutability and String Pool

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  More about JavaStrings * Immutable * String Pool Sujit Kumar Zenolocity LLC ©2013-2024
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  Immutable Strings • Strings1 = “hello”; • String s2 = “hi”; • “hello” and “hi” are the objects in memory. These are immutable => cannot change. • But s1 and s2 are references to the objects. • References are mutable. They can be changed to point to different objects (of same type or inherited type). • s1 = “hola!”; • Now s1 is pointing to a different object, that is allowed.
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  Immutable Strings • “hello”object itself cannot change. “hi” cannot change. “hola” cannot change. • But references (pointers) to these objects can change.
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  String Pool forString literals • Special storage area in Java heap. • When a string literal is used to create a string and if the string already exists in the pool, the reference of the existing string will be returned. • Will NOT create a new object and return its reference. • Saves valuable heap memory. • String s1 = “hello”; • String s2 = “hello”; • Only 1 object with 5 chars is created by the above 2 statements. • Both s1 and s2 point to that shared object. • s1 == s2 will evaluate to true.
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  String Object • Creatinga String object with the new operator WILL NOT place it in the STRING POOL. • String s3 = new String(“hello”); • String s4 = new String(“hello”); • s3 == s4 will be false • Why String Pool ? -- Strings are very commonly used. Sharing Strings from a pool saves valuable heap memory.
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  CharSequence • Interface • Representsa readable sequence of char values. • Important classes implementing this interface are String, StringBuffer and StringBuilder.
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  Why are Stringsimmutable? • If string is not immutable, changing a given string in the string pool with one reference will lead to the wrong value for all the other references to the same string in the string pool. • Hashcode of String frequently used. Can be cached & hence more efficient. No need to recalculate it each time it is used. • Removes security threats when accessing network connections or files and also while accessing classes via the Reflection API.